1. Technical Field
The present disclosure relates generally to discharge lamps and, more particularly, to a discharge lamp for use in applications, such as tanning or technical lighting, wherein the lamp includes a vitreous tube having overlaid fluorescent coatings applied on an interior thereof to effectuate a desired intensity distribution along the length of the lamp.
2. Background of the Related Art
Discharge lamps have been in existence for many decades. Discharge lamps consist primarily of an elongated vitreous tube having axially opposed end seals. These lamps are coated on the inside with phosphor powders that fluoresce when excited by ultraviolet light. Filament electrodes are mounted on the end seals of the tube and are connected to base pins that engage with the lamp housing. The elongated tube is filled with a rare gas, such as argon, and a drop of mercury.
Discharge lamps typically operate at a relatively low pressure. In operation, an alternating current is applied to the electrodes, which increases the electrode temperature and causes the emission of electrons therefrom. These electrons are accelerated by the voltage across the tube until they collide with the mercury atoms, causing them to be ionized and excited. When the mercury atoms return to their normal state, mercury spectral lines in both the visible and ultraviolet region are generated. The ultraviolet radiation excites the phosphor coating to luminance. The resulting output is not only much higher than that obtained from the mercury lines alone, but also results in a continuous spectrum with colors dependent upon the phosphors used. Typically, the intensity of the ultraviolet radiation emitted from the discharge lamp differs to some degree along the length of the lamp. However, such variation is generally not dramatic, nor are distinct regions of varying intensity created.
In certain lighting applications, such as tanning, it would be advantageous to provide or deliver distinct regions of ultraviolet radiation intensity. Moreover, in technical lighting applications, such as scanning or copying, it would be advantageous to provide a substantially uniform luminous intensity along the length of the lamp. Other needs and opportunities for improvement associated with a range of lighting applications will be apparent from the discussion that follows.
I. Tanning Applications
Since the late 1970s, the practice of tanning, defined as the darkening of one's skin through exposure to ultraviolet (UV) radiation, has increased in popularity in the United States. Each person's skin reacts differently to UV radiation exposure, with the reaction being dependent upon genetically determined factors, such as the amount of melanin pigment already in the skin naturally and the capability of the person's skin to produce additional melanin (facultative pigmentation).
Melanin is the dark pigment found in the retina, hair and skin, except for the palms of the hands, soles of the feet and lips. Without the protection afforded by the melanin pigment, a person's skin would burn when exposed to UV radiation. As stated above, the skin includes naturally occurring melanin pigment and produces additional melanin. Melanin is produced by special cells called melanocytes, which are located deep within the outer layer of the skin. When the melanocytes are stimulated by ultraviolet light, they utilize an amino acid called tyrosine to produce the pigment melanin. Since the melanin pigment is only able to absorb ultraviolet light of approximately 260-320 nanometers, UVB radiation is needed to achieve melanin production. UVA radiation which has a wavelength of approximately 320-400 nanometers can formulate melanin, but only when there is enough photosensitizing material already in the skin to trigger a UVB reaction. With the presence of UVB, melanocytes are stimulated to divide, creating more pigment cells. During this time, the epidermis thickens to form additional protection, a condition referred to as acanthosis.
In the beginning stages of melanin production, the skin has very little melanin or radiation protection capabilities. As a result, UVA radiation is not blocked by melanin pigments and, due to its longer wavelength, penetrates the skin deeper than UVB, causing damage to the corium. Damage to this layer of the epidermis hastens aging and destruction of collagen and connective tissue. A UVA burn can be much more damaging because it is not felt due to its deep penetration.
In order for the pigmentation process to be effective, melanin granules must be oxidized or darkened, which requires a high dose of long-wave UVA. Consequently, exposure to UVB radiation functions to create melanin pigment, while UVA exposure ensures the oxidation of the pigment. Together, the proper combined UV exposure operates to create a light-protection mechanism.
It is well recognized that to obtain the desired uniform tan, a person's facial region often requires the application of more intense radiation than the body region. This is due to the higher levels of melanin pigment present in the face, resulting from a more frequent exposure to the sun than the body. Prior attempts at designing a tanning chamber that provides a more uniform tan have included a lamp assembly that utilizes separate and distinct bulbs in the facial region. More specifically, higher intensity metal halide bulbs are positioned in the facial region and lower intensity bulbs extend over the body.
There is a need therefore, for a discharge lamp for use in applications such as tanning, wherein a single discharge lamp has multiple regions of varying ultraviolet radiation intensity along its length.
II. Technical Lighting Applications
Lighting applications which require a lamp to provide light having a controlled and/or specific luminous intensity or pattern are commonly termed “technical lighting” applications. Document scanning and photocopying are examples of technical lighting applications. These applications require the lamp to provide light having a substantially uniform luminous intensity along its length.
In scanning, a document to be digitally replicated is placed on a glass plate and the housing cover is closed. The scan head, which includes at least two mirrors, a lens, a filter, a charge coupled device (CCD) array and an exposure lamp, is moved slowly across the document by a belt that is attached to a stepper motor. The exposure lamp, which is often a fluorescent discharge lamp, is used to illuminate the document. The exposed image of the document is reflected by a first mirror into a second mirror and then onto the lens. The lens focuses the image through the filter onto the CCD array. The CCD array converts the reflected image into a data signal, which is then sent to a multifunctional circuit board for image processing (including enlarging, reducing, rotating, etc.). The uniformity of the document exposure by the lamp is critical to the quality of the scanned image. More specifically, the lamp must expose the document to be scanned to a uniform luminous intensity and therefore, the exposure lamp must provide a uniform output along its length.
It is generally well known that the intensity of the ultraviolet radiation and visible light emitted from a discharge lamp decreases from the center towards the ends thereof. The lack of uniformity along the length of the lamp is disadvantageous when the lamp is used in technical lighting applications such as document scanning and photocopying.
U.S. Pat. No. 3,717,781 to Sadoski et al. discloses a discharge lamp which is configured to improved the uniformity of the light output along its length. The Sadoski et al. lamp is an aperture fluorescent lamp. Aperture lamps are those lamps whose brightness is increased by scraping away the phosphor coating along a narrow strip extending the entire length of the lamp. When a reflective layer, such as titanium dioxide, is interposed between the lamp glass wall and the fluorescent phosphor coating, and a narrow strip of this material is scraped away, the brightness of the surface is greatly enhanced. The light output profile of such an aperture lamp also shows a decrease in light intensity at the ends thereof, similar to conventional fluorescent lamps. Sadoski et al. configured the lamp such that the aperture was made wider at the ends of the lamp than at the center, so as to reduce the variation in the light output profile.
The patent literature also includes disclosures concerning aperture lamps wherein the entire outside of the glass tube envelope, except for the aperture, is covered with a reflecting member whose surface area increases towards the ends of the tube. See e.g., U.S. Pat. No. 3,767,956 to Bauer.
As stated above, removing a portion or narrow strip of the phosphor coating creates the aperture. A disadvantage associated with aperture lamps is that removal of the phosphor coating to create the aperture is expensive. Additionally, the brightness of the lamp is dependent on the amount of phosphor coating. Therefore, removal of the phosphor coating beyond a narrow strip reduces the overall brightness of the lamp.
There is a need therefore, for discharge lamps for use in technical lighting applications, such as photocopying or scanning, which cost effectively and without degradation in lamp brightness emit light having a substantially uniform luminous intensity along the length thereof.
There is also need for a method of making discharge lamps for use in applications such as tanning or technical lighting, which, cost effectively and without degradation in lamp brightness, adapts a discharge lamp to emit light having a desired intensity distribution along the length of the lamp. More specifically, there is a need for a method of manufacturing discharge lamps which can readily configure the lamp to provide a luminous output that includes distinct regions of varying intensity along the length of the lamp or an output which is substantially uniform along the length thereof.